nix-super/src/fstate.hh

#ifndef __FSTATE_H
#define __FSTATE_H

extern "C" {
#include <aterm2.h>
}

#include "hash.hh"
#include "store.hh"

using namespace std;


/* \section{Abstract syntax of Nix file system state expressions}

   A Nix file system state expression, or FState, describes a
   (partial) state of the file system.

     Slice : [Id] * [(Path, Id, [Id])] -> FState

   (update)
   Path(path, content, refs) specifies a file object (its full path
   and contents), along with all file objects referenced by it (that
   is, that it has pointers to).  We assume that all files are
   self-referential.  This prevents us from having to deal with
   cycles.

     Derive : [(Path, Id)] * [FStateId] * Path * [(String, String)] -> FState

   (update)
   Derive(platform, builder, ins, outs, env) specifies the creation of
   new file objects (in paths declared by `outs') by the execution of
   a program `builder' on a platform `platform'.  This execution takes
   place in a file system state given by `ins'.  `env' specifies a
   mapping of strings to strings.

   A FState expression is in {\em $f$-normal form} if all Derive nodes
   have been reduced to File nodes.

   DISCUSSION: the idea is that a Regular/Directory is interchangeable
   with its CHash.  This would appear to break referential
   transparency, e.g., Derive(..., ..., [...CHash(h)...], ...) can
   only be reduced in a context were the Regular/Directory equivalent
   of Hash(h) is known.  However, CHash should be viewed strictly as a
   shorthand; that is, when we export an expression containing a
   CHash, we should also export the file object referenced by that
   CHash.

*/

typedef ATerm FState;
typedef ATerm Content;

typedef list<FSId> FSIds;


struct SliceElem
{
    string path;
    FSId id;
    FSIds refs;
};

typedef list<SliceElem> SliceElems;

struct Slice
{
    FSIds roots;
    SliceElems elems;
};


/* Return a canonical textual representation of an expression. */
string printTerm(ATerm t);

/* Throw an exception with an error message containing the given
   aterm. */
Error badTerm(const format & f, ATerm t);

/* Hash an aterm. */
Hash hashTerm(ATerm t);

/* Read an aterm from disk, given its id. */
ATerm termFromId(const FSId & id, string * p = 0);

/* Write an aterm to the Nix store directory, and return its hash. */
FSId writeTerm(ATerm t, const string & suffix, string * p = 0);

/* Register a successor. */
void registerSuccessor(const FSId & id1, const FSId & id2);


/* Normalise an fstate-expression, that is, return an equivalent
   Slice. */
Slice normaliseFState(FSId id);

/* Realise a Slice in the file system. */
void realiseSlice(const Slice & slice);

/* Get the list of root (output) paths of the given
   fstate-expression. */
Strings fstatePaths(const FSId & id, bool normalise);

/* Get the list of paths referenced by the given fstate-expression. */
StringSet fstateRefs(const FSId & id);


#endif /* !__FSTATE_H */